Johnson, 22, then knifed Seyi Ogunyemi, 16, six times while he lay helpless on the ground, before fleeing the scene covered in blood.

Today a judge ruled Johnson must serve at least 24 years in jail, as he became the first killer to be brought to justice using pioneering dog DNA technology.

He was convicted of murder after forensic analysis proved to a billion-to-one certainty that blood found on him and at the scene came from the dangerous animal.

Seyi and his friend Hurui Hiyabu, 17, were set upon by a large group of youths and two dogs in Larkhall Park, south London, last April, the Old Bailey heard.

Prosecutor Brian Altman QC told jurors the case was “unique” because the pets were used as weapons.

One witness described the attackers as “vicious” and said they were acting like a “pack of wild animals”.

Johnson, of south Lambeth, south London, was found guilty by a jury of Seyi’s murder and the attempted murder of his friend - said to have been lucky to survive after he was stabbed nine times.

Judge Christopher Moss told him: “You used two fearsome weapons. The first was your pitbull-cross dog, which I have no doubt you had trained to attack and bring down your prey.

“The second was the knife with which you stabbed Seyi Ogunyemi to death.”

Relatives of the victim hugged each other after the sentence while friends and family of the killer cried “innocent” from the public gallery.

The judge ordered dog Tyson to be forfeited to the police. It faces being put down.

The judge told Johnson: “This court has not heard any evidence to explain the enmity which obviously existed between you and your victims, save that you were plainly members of rival gangs which each claimed dominion of the south London streets in which you lived.

“This is a social problem that blights many urban neighbourhoods in our cities.

“The courts cannot cure the problem. All they can do is react firmly and decisively and impose severe punishment when murderous young men such as you are brought to justice before them.

“It must be hoped that the message will go out that gang-related violence such as this will not be tolerated, and - as is tragically so often the case - when such violence results in injury or death, the perpetrators will be pursued and if convicted face a substantial sentence.”

Seyi, who was of slight build and suffered from Crohn’s disease, was brought to the ground and mauled by Tyson - an adult male Staffordshire bull terrier/bull mastiff cross - as he tried to escape over a fence.

The boy stood no chance once he was in the dog’s ferocious grip and was then knifed repeatedly by its owner.

Johnson was arrested as he fled the scene, bare-chested and covered in blood, some of it human and some of it canine.

He had complied with a court order in late 2007 to have Tyson chipped, tattooed and insured but was also supposed to keep him muzzled and on a lead at all times while in public.

New technology, used for the first time, proved to a billion-to-one certainty that the dog blood came from Tyson, who had been knifed during the melee. The rest came from the teenage murder victim.

Police hailed the DNA technique, which had just been developed at the time of the murder, as a “hugely powerful investigative tool”.

They said they hoped it would deter others from using dogs as weapons.

Detective Inspector Mick Norman said: “This horrific attack was committed on a very slight teenage boy who stood no chance of defending himself.

“The fact that Chrisdian Johnson also ordered dogs to take part in the attack illustrates his sickening attitude to violence.

“The advances in dog DNA and forensic work now means that anyone who owns a dog and uses it to attack people can be identified and prosecuted.”